Experimental studies of fabricated eddy current probe / Ilham Mukriz Zainal Abidin

Eddy current testing (ET) is one of the Non destructive testing (NDT) techniques for the detection and evaluation of surface and sub-surface defects in electrically conducting materials. This technique is the most effective technique for the assessment of heat exchanger tubes in monitoring the integrity of a heat exchanger system. In performing the eddy current inspection , probe is the most important component in acquiring information from the heat exchanger system. It is the main factor that determines the success of an eddy current testing for optimum and reliable inspection results. This research work covers the experimental and numerical approach in fabricating an eddy current probe for tube inspection . The aim of the research is to study the physics and engineering parameters which can enable us to produce eddy current probes with the focus of studying the probe coil configuration that affect its sensitivity and resolution for eddy current testing. With the achievement in fabricating an eddy current probe that fulfill the requirement of code and standards for tube inspection, the work is proceed in the study of the effect of inter-coil spacing and coil width to the probe sensitivity and resolution. The sensitivity and resolution of the fabricated probes have been studied by measurement of Vpp values and signal phase separation between internal groove defect (10) & external groove defect (00) wall loss at different intercoil spacing and different coil width. The results obtained by both experimental and numerical work have shown that with reduced inter-coil spacing and coil width, the sensitivity and the resolution of the fabricated probes will be increased thus giving a better inspection reliability and performance. This behavior is mainly attributed to the physical parameter of the probe geometry. With reduced spacing and coil width, the eddy current density becomes denser in the test specimen at a specific region. This reflects that the resistance for the eddy currents to flow in the sample is reduced and the phase will be increased. In addition, with reduced inter-coil spacing between the two coils, the mutual impedance of the two coils will become dominant thus a denser eddy current will be induced in the sample. In eddy current testing, defect detection is based on how the eddy current is disturbed in the sample , with more induced current , more current will be affected by the presence of a defect thus increase the sensitivity and the resolution of the probe. In this study, there are good agreement between the experimental data and numerical model in determining the reliable eddy current probe for engineering application

Modelling and experimental investigation of eddy current distribution for angular defect characterisation

Current industrial requirements for nondestructive testing demand defect quantification rather than simple defect detection. This is not a simple task as defects in components, such as cracks, rarely have a simple geometrical shape. Therefore, the influence of defect shape and orientation and its effect on the inspection results needs to be addressed to avoid misinterpretation of the response signals and for a quantitative characterisation of defects. Finite element method (FEM) numerical simulations for eddy current non-destructive evaluation (ECNDE) can provide information on how the induced eddy current interacts with defects and the effect of defect shape and geometry towards the results. Through the analysis of the simulation results, links can be established between the measurements and information relating to the defect, such as 3-D shape, size and location, which facilitates not only forward problem but also inverse modelling involving experimental system specification and configuration; and pattern recognition for 3-D defect information. This work provides a study of the characterisation of angular defects through the technique of visualisation and mapping of magnetic field distribution for pulsed eddy current (PEC) and temperature distribution for PEC thermography. 3-D FEM simulations are utilised to provide the guidelines for experimental designs and specifications; understanding of the underlying physics surrounding a particular defect; and means for features extraction from the acquired responses. Through the study, defect Quantitative Non-destructive Evaluation (QNDE) has been established using the features extracted from the mapping by taking into consideration the angular characteristic of defect in the inspection results. Experimental investigations are then performed to verify the simulation results and the feasibility of the proposed techniques and extracted features to be used in acquiring information about the angular defect. The work concludes that the technique of mapping the resultant distribution from the interaction of eddy currents and defects has provided the vital information needed for defect characterisation. Features extracted from the mapping via numerical investigations have provided the means for the QNDE of angular defects. The work shows that the technique and features introduced has provided an alternative way for defect characterisation and QNDE, which also can be extended its application to other industrial components and research field.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Simulation and Experimental Investigation of Pulsed Eddy Current Technique for Defect Evaluation

Pulsed eddy current (PEC) is an advanced Non-Destructive Testing (NDT) technique that uses transient waveform for their coil excitation. It has the advantage of gathering different depth information in a single excitation process, thus provide the solution towards the drawbacks by both single and multi-frequency eddy current testing. In this work, experimental and simulation investigations on PEC were conducted to establish the correlation between PEC signals and different defect i.e. crack, depths. Initial FEM simulation work has provided understanding of the underlying phenomena of the PEC results through the visualisation of the induced eddy current and defect interaction in the SS 304 sample. Features from the differential magnetic field transient profiles have provided information of the crack in terms of its depth and location. The understanding of the PEC responses and results can be extended for future work in quantitative evaluation of defect in conductive samples by PEC

Modelling of scanning pulsed eddy current testing of normal and slanted surface cracks

Thanks to its wide bandwidth, pulsed eddy current (PEC) has attracted researchers of various backgrounds in the attempt to exploit its benefits in Non-destructive Testing (NDT). The ability of modelling PEC problems would be a precious tool in this attempt as it would help improve the understanding of the interaction between the transient magnetic field and the specimen, among others. In this work, a Finite Element Modelling (FEM) has been developed and experimental test data have been gathered for its validation. The investigated cases were simulated surface cracks of different sizes and angles. The study involved looking at time-domain PEC signals at different spatial distances from the cracks’ faces, which would particularly be useful for modelling scanning PEC probes. The obtained results show a good agreement between the FEM and experiment, demonstrating that the modelling technique can be used with confidence for solving similar problems. In addition, the extracted features from signals were also studied to discover the influence of crack geometries to the PEC responses

Quantitative evaluation of crack depths and angles for pulsed eddy current non-destructive testing

Cracks with inclination angles may potentially cause damage to a larger region in the tested structures. Their characterization, in terms of depth and angle, is therefore paramount for ensuring the integrity of the specimen under test. This study extracts features from Pulsed eddy current (PEC) signals obtained in a linear scan, perpendicular to the simulated surface cracks. The novel features extracted, termed skewness, LLS and LSmax, are capable of defining crack depth and inclination angles simultaneously. Multiple linear regression (MLR) was built to perform depth prediction, and the pre-determined depths were used in the hierarchical linear model (HLM) for angle prediction. The results were then compared with depth and angle prediction using artificial neural network (ANN). Better reliability of the ANN model with recorded RMSE of 0.198mm and 2.903° in depth and angle prediction are highlighted. ANN is favourable in handling simultaneous prediction of crack depth and inclination angles, when using interdependent features. Meanwhile, HLM is still approved as a technique to provide a preliminary understanding of the crack parameters

Image-based feature extraction technique for inclined crack quantification using pulsed eddy current

Existing eddy current non-destructive testing (NDT) techniques generally do not consider the inclination angle of inclined cracks, which potentially harms a larger region of a tested structure. This work proposes the use of 2D scan images generated by using pulsed eddy current (PEC) non-destructive testing (NDT) technique in the quantification of the inclination and depth of inclined cracks. The image-based feature extraction technique effectively identifies the crack axis, which consequently enables extraction of features from the extracted linear scans. The technique extracts linear scans from the images to allow the extraction of three novel image-based features, namely the length of extracted linear scans (LLS), the linear scan skewness (LSS), and the highest value on linear scan (LSmax). The correlation of the three features to surface crack inclination angles and depths were analysed and found to be highly dependent on the crack depths, while only LLS and LSS are correlated to the crack inclination angles. © 2019, The Author(s)

Investigation of capabilities of electromagnetic tomography for pipeline imaging

In industrial process, pipeline leakage often only noticeable pipe condition is critical henceforth causing damages to its internal content. Therefore, pipeline inspection and monitoring work is highly demanding to take early precautions. Electromagnetic Tomography (EMT) is capable to produce images of the internal structure of an object by using external sensors without disrupting it. The coil sensors are placed around the object where the source coil transmits an oscillating magnetic field while the rest acts as receiver and measure the received signals. The measured signal provides the magnetic field distribution of the pipe, and will differ according to the materials’ passive electrical properties. In this paper, we investigate the capability of using EMT to identify metallic pipe openings by conducting a finite element analysis simulation study. The design and parameters of the EMT system, as well as the results of using the EMT model to detect various degree of metallic pipe openings is presented. The results confirm that the EMT imaging as a promising tool for inspection of metallic pipelines where the magnetic field of the investigated region differs according to the pipe opening and material of the pipeline

A Gradient-Field Pulsed Eddy Current Probe for Evaluation of Hidden Material Degradation in Conductive Structures Based on Lift-Off Invariance

Coated conductive structures are widely adopted in such engineering fields as aerospace, nuclear energy, etc. The hostile and corrosive environment leaves in-service coated conductive structures vulnerable to Hidden Material Degradation (HMD) occurring under the protection coating. It is highly demanded that HMD can be non-intrusively assessed using non-destructive evaluation techniques. In light of the advantages of Gradient-field Pulsed Eddy Current technique (GPEC) over other non-destructive evaluation methods in corrosion evaluation, in this paper the GPEC probe for quantitative evaluation of HMD is intensively investigated. Closed-form expressions of GPEC responses to HMD are formulated via analytical modeling. The Lift-off Invariance (LOI) in GPEC signals, which makes the HMD evaluation immune to the variation in thickness of the protection coating, is introduced and analyzed through simulations involving HMD with variable depths and conductivities. A fast inverse method employing magnitude and time of the LOI point in GPEC signals for simultaneously evaluating the conductivity and thickness of HMD region is proposed, and subsequently verified by finite element modeling and experiments. It has been found from the results that along with the proposed inverse method the GPEC probe is applicable to evaluation of HMD in coated conductive structures without much loss in accuracy

Finite element modelling of pulsed eddy current non-destructive testing for different defect natures

Pulsed eddy current (PEC) has attracted researchers of various backgrounds in the attempt to optimize the benefits of this method in Non-destructive Testing (NDT). With various means of approach in solving PEC problems, a correlation between Finite Element Modeling (FEM) and experimental study has to be done. Three cracks of different natures have been 3D modelled using COMSOL Multiphysics 5.2a and verified experimentally. The results involve taking the signals at different positions away from the middle of the opening of the cracks. Different trends employed by different natures of defect are also presented in the pape

Imaging of Subsurface Corrosion Using Gradient-Field Pulsed Eddy Current Probes with Uniform Field Excitation

A corrosive environment leaves in-service conductive structures prone to subsurface corrosion which poses a severe threat to the structural integrity. It is indispensable to detect and quantitatively evaluate subsurface corrosion via non-destructive evaluation techniques. Although the gradient-field pulsed eddy current technique (GPEC) has been found to be superior in the evaluation of corrosion in conductors, it suffers from a technical drawback resulting from the non-uniform field excited by the conventional pancake coil. In light of this, a new GPEC probe with uniform field excitation for the imaging of subsurface corrosion is proposed in this paper. The excited uniform field makes the GPEC signal correspond only to the field perturbation due to the presence of subsurface corrosion, which benefits the corrosion profiling and sizing. A 3D analytical model of GPEC is established to analyze the characteristics of the uniform field induced within a conductor. Following this, experiments regarding the imaging of subsurface corrosion via GPEC have been carried out. It has been found from the results that the proposed GPEC probe with uniform field excitation not only applies to the imaging of subsurface corrosion in conductive structures, but provides high-sensitivity imaging results regarding the corrosion profile and opening size